This invention relates to a process for coating printing cylinders with a layer of a liquid, especially for coating flexographic printing sleeves with infrared sensitive layers. Furthermore, the invention relates to an apparatus for this process and to a flexographic printing sleeve made by this process.
Flexographic printing forms are well known for use in relief printing and letterpress printing on a variety of substrates such as paper, corrugated board, films, foils, and laminates. Flexographic printing forms can be prepared from photopolymerizable elements which generally comprise a photopolymerizable layer of an elastomeric binder, a monomer, and a photoinitiator as main components, interposed between a support and a cover sheet or multilayer cover element. Upon imagewise exposure with actinic radiation through a photomask, the exposed areas of the photopolymerizable layer are insolubilized. Treatment with a suitable solvent removes the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing. Such materials are described in U.S. Pat. No. 4,323,637; U.S. Pat. No. 4,427,759; and U.S. Pat. No. 4,894,315.
Digital methods and associated recording materials that do not require a photomask have been developed and are described in WO 94/03838, WO 94/03839, WO 96/16356, and EP 0767 407. Such recording materials comprise a conventional photopolymerizable layer, as previously described, and additionally a layer capable of forming an integrated photomask. The additional layer is sensitive to infrared radiation and opaque to actinic radiation, a so-called infrared sensitive layer. This infrared sensitive layer is imaged digitally, whereby the infrared sensitive material is imagewise vaporized or transferred to a superposed film. Subsequent overall exposure of the photopolymerizable element through the resulting integrated photomask, washing off unpolymerized areas and remaining areas of the infrared sensitive layer, and drying the element yield a flexographic printing form.
These digital methods are used for the preparation of flexographic printing forms in sheet form or in cylindrical form. For flexographic printing forms which are cylindrical, the photopolymerizable layer is on a cylindrical shaped carrier, a so called sleeve. Sleeves are readily and repeatably mounted and dismounted from print drums. Such sleeves are described in EP 0 696 247. Besides the advantages associated with printing production for easily mountable and dismountable printing forms, there are particular applications and advantages to using the printing form in cylindrical form. Continuous printing forms have applications in flexographic printing of continuous designs such as in wallpaper, decoration and gift wrapping paper. Furthermore, such cylindrical printing forms are well-suited for mounting on laser exposure equipment where it can replace a drum or be mounted on the drum for exposure by a laser.
For digital laser imaging, a cylindrical printing element must be coated with a thin layer sensitive to laser radiation used for the imaging process. Currently, coatings on elastomeric and soft flexographic printing cylinders or sleeves are done by two methods (a) ring coating and (b) spray coating.
Ring coating is a method which touches the cylinder or sleeve, therefore, it is usually only applied for hard and solid gravure printing cylinders. Trials to coat on soft and elastomeric flexographic printing sleeves have been tried in the past, but often damages and scratches the soft and tacky surfaces of the elastomeric flexographic printing sleeves. The problem is that the photopolymerizable layer on the sleeve is coated before UV exposure and processing, therefore, is unhardened and soft during coating and very sensitive for mechanical damages. In addition, is was found that the coating is uneven from top to bottom, with a lower coating weight at the top versus the bottom, due to upright standing of the printing cylinder and fluid flow down to the end of the sleeve during coating. This is a severe problem especially with slow drying coating liquids. Other disadvantages of ring coating are the unclean coating at the edges (of sleeve start and end), further the low yield (due to remaining coating liquid in the coating container) and the high cleaning effort after coating.
Consequently, a coating technology like spray coating is preferred for soft surfaces. However, spray coating of thin layers is difficult to achieve and needs special spray nozzles to cover the cylindrical sleeve with the coating uniformity requested for the laser imaging process. Due to the request for uniform coverage, the spray process has to be applied in several passes to obtain pinhole-free coatings. Especially, for small sleeve diameters the spray cone is much larger than the sleeve geometry; this leads to significant overspray and loss of spray liquid. At the edges (sleeve start and end) unclean overspraying is observed if the edges are not covered with a tape. Further, the flammability of the spray droplets has to be considered when flammable solvents are used for the coating. This can be a significant safety hazard and needs specific protection for the equipment.
Another coating technique that touches the surface of the printing cylinder is roller coating. A coating liquid is transported by a coating roll to the surface that is going to be coated, whereby the coating roll is in close contact with this surface or even pressed against it. A special roller coating method is described in U.S. Pat. No. 5,279,861. This method comprises such a roller coating wherein a rotating coating roll, immersed in the coating liquid, is contacted with a rotating printing cylinder and moved from one end of the printing cylinder to the other in such a manner that the coating roll is kept in contact with the printing cylinder and the coating liquid is coated on the printing cylinder.
But this method is only applicable to hard and solid gravure printing cylinders. Flexographic printing cylinders can not be coated by this process because their tacky and soft photopolymerizable surfaces can not be coated without any scratches or surface defects. These defects would later show up as defects in the printed image, making the printing cylinder unusable.
Therefore, it was an objective of the present invention to provide a coating process that does not damages the surface of the printing cylinder to be coated and which provides uniform coatings.
This objective is solved by a process for coating a printing cylinder with a layer of a liquid, comprising the steps of: (a) forming a fluid film of the liquid on a surface of a coating roll; (b) positioning the surface of the coating roll in a predetermined distance from an outer surface of the printing cylinder such that the fluid film contacts the outer surface and a coating gap between the outer surface of the printing cylinder and the surface of the coating roll is formed; (c) simultaneously rotating and moving the coating roll relative to the printing cylinder in such a manner that the printing cylinder is coated with the liquid layer; and (d) drying the liquid layer to form the coated printing cylinder.
In another embodiment, the invention is directed to an apparatus to perform this process.
In a further embodiment, the invention is directed to a flexographic printing sleeve made by such a process.